We propose to test a hypothesis concerning the mechanism of coupling between the T-system and sarcoplasmic reticulum of skeletal muscle. The T-system is an invaginated system of tubules which conducts the action potential radially into the muscle fiber. The SR is a separate membrane system which is responsible for the storage and release of calcium, which calcium in turn is the chemical trigger for contraction. Our hypothesis is that the coupling between T-system and SR is electrical, that ionic current flows from tubular lumen to SR lumen during a normal twitch. This hypothesis will be tested by a study of electrical current flow accompanying depolarization, by study of structural changes accompanying contraction, and by attempts to detect movement of a marker from t-system to SR. Much other work will be done directly relating to cardiac muscle, including publication of the equivalent circuit of sheep Purkinje Fibers, and of aggregates of cardiac cells grown in tissue culture.